02 Fakultät Bau- und Umweltingenieurwissenschaften
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/3
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Item Open Access Obstacles, interfacial forms, and turbulence : a numerical analysis of soil-water evaporation across different interfaces(2020) Coltman, Edward; Lipp, Melanie; Vescovini, Andrea; Helmig, RainerExchange processes between a turbulent free flow and a porous media flow are sensitive to the flow dynamics in both flow regimes, as well as to the interface that separates them. Resolving these complex exchange processes across irregular interfaces is key in understanding many natural and engineered systems. With soil-water evaporation as the natural application of interest, the coupled behavior and exchange between flow regimes are investigated numerically, considering a turbulent free flow as well as interfacial forms and obstacles. Interfacial forms and obstacles will alter the flow conditions at the interface, creating flow structures that either enhance or reduce exchange rates based on their velocity conditions and their mixing with the main flow. To evaluate how these interfacial forms change the exchange rates, interfacial conditions are isolated and investigated numerically. First, different flow speeds are compared for a flat surface. Second, a porous obstacle of varied height is introduced at the interface, and the effects the flow structures that develop have on the interface are analyzed. The flow parameters of this obstacle are then varied and the interfacial exchange rates investigated. Next, to evaluate the interaction of flow structures between obstacles, a second obstacle is introduced, separated by a varied distance. Finally, the shape of these obstacles is modified to create different wave forms. Each of these interfacial forms and obstacles is shown to create different flow structures adjacent to the surface which alter the mass, momentum, and energy conditions at the interface. These changes will enhance the exchange rate in locations where higher velocity gradients and more mixing with the main flow develop, but will reduce the exchange rate in locations where low velocity gradients and limited mixing with the main flow occur.Item Open Access An adaptive hybrid vertical equilibrium/full‐dimensional model for compositional multiphase flow(2022) Becker, Beatrix; Guo, Bo; Buntic, Ivan; Flemisch, Bernd; Helmig, RainerEfficient compositional models are required to simulate underground gas storage in porous formations where, for example, gas quality (such as purity) and loss of gas due to dissolution are of interest. We first extend the concept of vertical equilibrium (VE) to compositional flow, and derive a compositional VE model by vertical integration. Second, we present a hybrid model that couples the efficient compositional VE model to a compositional full‐dimensional model. Subdomains, where the compositional VE model is valid, are identified during simulation based on a VE criterion that compares the vertical profiles of relative permeability at equilibrium to the ones simulated by the full‐dimensional model. We demonstrate the applicability of the hybrid model by simulating hydrogen storage in a radially symmetric, heterogeneous porous aquifer. The hybrid model shows excellent adaptivity over space and time for different permeability values in the heterogeneous region, and compares well to the full‐dimensional model while being computationally efficient, resulting in a runtime of roughly one‐third of the full‐dimensional model. Based on the results, we assume that for larger simulation scales, the efficiency of this new model will increase even more.Item Open Access Modeling of two phase flow in a hydrophobic porous medium interacting with a hydrophilic structure(2022) Michalkowski, Cynthia; Weishaupt, Kilian; Schleper, Veronika; Helmig, RainerFluid flow through layered materials with different wetting behavior is observed in a wide range of applications in biological, environmental and technical systems. Therefore, it is necessary to understand the occuring transport mechanisms of the fluids at the interface between the layered constituents. Of special interest is the water transport in polymer electrolyte membrane fuel cells. Here, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. This is crucial to choose optimal operating conditions and improve the overall cell performance. Pore-scale modeling of gas diffusion layers (GDLs) and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the hydrophobic porous GDL and the hydrophilic gas distributor, a particular challenge is the combination and interaction of the different material structures and wetting properties at the interface and its influence on the flow. In this paper, a modeling approach is presented which captures the influence of a hydrophilic domain on the flow in a hydrophobic porous domain at the interface between the two domains. A pore-network model is used as the basis of the developed concept which is extended to allow the modeling of mixed-wet interactions at the interface. The functionality of the model is demonstrated using basic example configurations with one and several interface pores and it is applied to a realistic GDL representation in contact with a channel-land structured gas distributor.Item Open Access A new simulation framework for soil-root interaction, evaporation, root growth, and solute transport(2018) Koch, Timo; Heck, Katharina; Schröder, Natalie; Class, Holger; Helmig, RainerWe have developed a general model concept and a flexible software framework for the description of plant-scale soil-root interaction processes including the essential fluid mechanical processes in the vadose zone. The model was developed in the framework of non-isothermal, multiphase, multicomponent flow and transport in porous media. The software is an extension of the open-source porous media flow and transport simulator DuMux to embedded mixed-dimensional coupled schemes. Our coupling concept allows us to describe all processes in a strongly coupled form and adapt the complexity of the governing equations in favor of either accuracy or computational efficiency. We have developed the necessary numerical tools to solve the strongly coupled nonlinear partial differential equation systems that arise with a locally mass conservative numerical scheme even in the context of evolving root architectures. We demonstrate the model concept and its features, discussing a virtual hydraulic lift experiment including evaporation, root tracer uptake on a locally refined grid, the simultaneous simulation of root growth and root water uptake, and an irrigation scenario comparing different models for flow in unsaturated soil. We have analyzed the impact of evaporation from soil on the soil water distribution around a single plant’s root system. Moreover, we have shown that locally refined grids around the root system increase computational efficiency while maintaining accuracy. Finally, we demonstrate that the assumptions behind the Richards equation may be violated under certain conditions.Item Open Access Two-phase flow dynamics at the interface between GDL and gas distributor channel using a pore-network model(2022) Michalkowski, Cynthia; Veyskarami, Maziar; Bringedal, Carina; Helmig, Rainer; Schleper, VeronikaFor improved operating conditions of a polymer electrolyte membrane (PEM) fuel cell, a sophisticated water management is crucial. Therefore, it is necessary to understand the transport mechanisms of water throughout the cell constituents especially on the cathode side, where the excess water has to be removed. Pore-scale modeling of diffusion layers and gas distributor has been established as a favorable technique to investigate the ongoing processes. Investigating the interface between the cathode layers, a particular challenge is the combination and interaction of the multi-phase flow in the porous material of the gas diffusion layer (GDL) with the free flow in the gas distributor channels. The formation, growth and detachment of water droplets on the hydrophobic, porous surface of the GDL have a major influence on the mass, momentum and energy exchange between the layers. A dynamic pore-network model is used to describe the flow through the porous GDL on the pore-scale. To capture the droplet occurrence and its influence on the flow, this dynamic two-phase pore-network model is extended to capture droplet formation and growth at the surface of the GDL as well as droplet detachment due to the gas flow in the gas distributor channels. In this article, the developed model is applied to single- and multi-tube systems to investigate the general drop behavior. These rather simple test-cases are compared to experimental and numerical data available in the literature. Finally, the model is applied to a GDL unit cell to analyze the interaction between two-phase flow through the GDL and drop formation at the interface between GDL and gas distributor channel.Item Open Access Estimation of capillary‐associated NAPL‐water interfacial areas for unconsolidated porous media by kinetic interface sensitive (KIS) tracer method(2023) Tatomir, Alexandru; Gao, Huhao; Abdullah, Hiwa; Pötzl, Christopher; Karadimitriou, Nikolaos; Steeb, Holger; Licha, Tobias; Class, Holger; Helmig, Rainer; Sauter, MartinBy employing kinetic interface sensitive (KIS) tracers, we investigate three different types of glass‐bead materials and three natural porous media systems to quantitatively characterize the influence of the porous‐medium grain‐, pore‐size and texture on the specific capillary‐associated interfacial area (FIFA) between an organic liquid and water. By interpreting the breakthrough curves (BTCs) of the reaction product of the KIS tracer hydrolysis, we obtain a relation for the specific IFA and wetting phase saturation. The immiscible displacement process coupled with the reactive tracer transport across the fluid-fluid interface is simulated with a Darcy‐scale numerical model. Linear relations between the specific capillary‐associated FIFA and the inverse mean grain diameter can be established for measurements with glass beads and natural soils. We find that the grain size has minimal effect on the capillary‐associated FIFA for unconsolidated porous media formed by glass beads. Conversely, for unconsolidated porous media formed by natural soils, the capillary‐associated FIFA linearly increases with the inverse mean grain diameter, and it is much larger than that from glass beads. This indicates that the surface roughness and the irregular shape of the grains can cause the capillary‐associated FIFA to increase. The results are also compared with the data collected from literature, measured with high resolution microtomography and partitioning tracer methods. Our study considerably expands the applicability range of the KIS tracers and enhances the confidence in the robustness of the method.Item Open Access Influence of radiation on evaporation rates : a numerical analysis(2020) Heck, Katharina; Coltman, Edward; Schneider, Jana; Helmig, RainerWe present a fully coupled soil‐atmosphere model that includes radiation in the energy balance of the coupling conditions between the two domains. The model is able to describe evaporation processes under the influence of turbulence, surface roughness, and soil heterogeneities and focuses specifically on the influence of radiation on the mass and energy transport across the soil‐atmosphere interface. It is shown that evaporation rates are clearly dominated by the diurnal cycle of solar irradiance. During Stage‐I evaporation maximum temperatures are regulated due to evaporative cooling, but after a transition into Stage‐II evaporation, temperatures rise tremendously. We compare two different soil types, a coarser, sandy soil and a finer, silty soil, and analyze evaporation rates, surface temperatures, and net radiation for three different wind conditions. The influence of surface undulations on radiation and evaporation is analyzed and shows that radiation can lead to different local drying patterns in the hills and the valleys of the porous medium, depending on the height of the undulations and on the direction of the Sun. At last a comparison of lysimeter measurement data to the numerical examples shows a good match for measured and calculated radiation values but evaporation rates are still overestimated in the model. Possible reasons for the discrepancy between measurement and model data are analyzed and are found to be uncertainties about the parameters close to the interface, which are decisive for determining evaporation rates.Item Open Access Investigation of different throat concepts for precipitation processes in saturated pore-network models(2024) Schollenberger, Theresa; Wolff, Lars von; Bringedal, Carina; Pop, Iuliu Sorin; Rohde, Christian; Helmig, RainerThe development of reliable mathematical models and numerical discretization methods is important for the understanding of salt precipitation in porous media, which is relevant for environmental problems like soil salinization. Models on the pore scale are necessary to represent local heterogeneities in precipitation and to include the influence of solution-air-solid interfaces. A pore-network model for saturated flow, which includes the precipitation reaction of salt, is presented. It is implemented in the open-source simulator DuMu X. In this paper, we restrict ourselves to one-phase flow as a first step. Since the throat transmissibilities determine the flow behaviour in the pore network, different concepts for the decreasing throat transmissibility due to precipitation are investigated. We consider four concepts for the amount of precipitation in the throats. Three concepts use information from the adjacent pore bodies, and one employs a pore-throat model obtained by averaging the resolved pore-scale model in a thin-tube. They lead to different permeability developments, which are caused by the different distribution of the precipitate between the pore bodies and throats. We additionally apply two different concepts for the calculation of the transmissibility. One obtains the precipitate distribution from analytical assumptions, the other from a geometric minimization principle using a phase-field evolution equation. The two concepts do not show substantial differences for the permeability development as long as simple pore-throat geometries are used. Finally, advantages and disadvantages of the concepts are discussed in the context of the considered physical problem and a reasonable effort for the implementation and computational costs.Item Open Access Modeling and analysis of droplet evaporation at the interface of a coupled free-flow-porous medium system(2024) Veyskarami, Maziar; Bringedal, Carina; Helmig, RainerEvaporation of droplets formed at the interface of a coupled free-flow–porous medium system enormously affects the exchange of mass, momentum, and energy between the two domains. In this work, we develop a model to describe multiple droplets’ evaporation at the interface, in which new sets of coupling conditions including the evaporating droplets are developed to describe the interactions between the free flow and the porous medium. Employing pore-network modeling to describe the porous medium, we take the exchanges occurring on the droplet-pore and droplet-free-flow interfaces into account. In this model, we describe the droplet evaporation as a diffusion-driven process, where vapor from the droplet surface diffuses into the surrounding free flow due to the concentration gradient. To validate the model, we compare the simulation results for the evaporation of a single droplet in a channel with experimental data, demonstrating that our model accurately describes the evaporation process. Then, we examine the impact of free-flow and porous medium properties on droplet evaporation. The results show that, among other factors, velocity and relative humidity in the free-flow domain, as well as pore temperature in the porous medium, play key roles in the droplet evaporation process.Item Open Access Evaporation-driven density instabilities in saturated porous media(2022) Bringedal, Carina; Schollenberger, Theresa; Pieters, G. J. M.; Duijn, C. J. van; Helmig, RainerSoil salinization is a major cause of soil degradation and hampers plant growth. For soils saturated with saline water, the evaporation of water induces accumulation of salt near the top of the soil. The remaining liquid gets an increasingly larger density due to the accumulation of salt, giving a gravitationally unstable situation, where instabilities in the form of fingers can form. These fingers can, hence, lead to a net downward transport of salt. We here investigate the appearance of these fingers through a linear stability analysis and through numerical simulations. The linear stability analysis gives criteria for onset of instabilities for a large range of parameters. Simulations using a set of parameters give information also about the development of the fingers after onset. With this knowledge, we can predict whether and when the instabilities occur, and their effect on the salt concentration development near the top boundary.